1. Field of the Invention
The present invention relates to a luminance signal and chrominance signal separation filter and particularly relates to a filter for separating a luminance signal and a chrominance singal from a composite color television signal of a standard system.
2. Description of the Prior Art
In a current standard television system, luminance information and chrominance information are transmitted as a composite signal having multiplexed frequencies and accordingly, it is necessary for the receiver to separate the information thus transmitted into a luminance signal and a chrominance signal correctly.
A composite color television signal S of the NTSC system is a composite signal including a luminance signal Y and a chrominance signal C which is obtained by applying two-phase modulation to two color-difference signals U and V (or I and Q) at a chrominance subcarrier frequency f.sub.sc, as shown in the following equation (1). EQU S=Y+C=Y+Usin (2.pi.f.sub.sc t)+Vcos (2.pi.f.sub.sc t) (1)
Assuming that the frame frequency f.sub.F is 30 Hz, the field frequency f.sub.V is 60 Hz and the horizontal scanning frequency f.sub.H is 15.75 KHz, these frequencies and the chrominance subcarrier frequency f.sub.sc have a relation as shown in the following equation (2). ##EQU1## As a result, if a composite color television signal of the NTSC system is synchronized and sampled at a sampling frequency f.sub.S which is four times greater than the chrominance subcarrier frequency f.sub.sc, the series of signal samples is arranged two-dimensionally on the screen as shown in FIG. 6. More specifically, four samples are taken for each cycle of the chrominance subcarrier with the phase of the chrominance signal C being inverted by 180.degree. for each line. In FIG. 6, the triangular and quadrangular symbols represent sample points; Y represents a luminance signal; C and C' represent respectively chrominance signals; and U and V represent respectively color-diffrerence signals.
On the other hand, a composite color television signal P of the PAL system is represented by the following equation (3): EQU P=Y+Usin (2.pi.f'.sub.sc t).+-.Vcos (2.pi.f'.sub.sc t) (3)
where f'.sub.sc represents a chrominance subcarrier frequency. In this equation (3), the plus and minus signs are selected alternately in a manner in which the plus sign + is selected on each even-numbered scanning line and the minus sign- is selected on each odd-numbered scanning line. In other words, the component V is inverted for each scanning line.
Assuming that the frame frequency f'.sub.F is 25 Hz, the field frequency f'.sub.V is 50 Hz and the horizontal scanning frequency f.sub.H is 15.625 KHz, these frequencies and the chrominance subcarrier frequency f'.sub.sc have a relation as indicated in the following equation (4). ##EQU2## More specifically, the chrominance subcarrier frequency f'.sub.sc and the horizontal scanning frequency f.sub.H are in a 1/4 line offset relation. As a result, the series of signal sample obtained by synchronizing and sampling the composite color television signal of the PAL system at a sampling frequency f'.sub.s which is four times the chrominance subcarrier frequency f'.sub.sc, is arranged two-dimensionally on the screen as shown in FIG. 7. More specifically, the phase of the chrominance signal becomes the same repeatedly in a four line cycle. In FIG. 7, the triangular and quadrangular symbols and the reference characters Y, C, C', U and V have the same meanings as in FIG. 6.
Generally, in order that a composite color television signal may have compatibility with a monochromatic television signal, the frequency of a chrominance signal of the composite color television signal is multiplexed in a manner in which the spectrum is frequency interleaved within the bandwidth of a luminance signal of the composite color television signal. Consequently, a television receiver needs to have a YC separation filter for correctly separating the luminance signal Y and the chrominance signal C.
An example of a conventional luminance signal and chrominance signal separation filter of this kind is shown in FIG. 1. Referring to FIG. 1, a series of signal sample 101 of a composite color television signal, which is sampled in synchronism with a chrominance subcarrier at a sampling frequency, is applied to a one-line delay circuit 1. The one-line delay circuit 1 delays the series of sampled signal 101 by one line and provides a one-line delayed signal 102. The one-line delayed signal 102 is supplied to another one-line delay circuit 2, which delays the one-line delayed signal 102 by one line and provides a two-line delayed signal 103. A vertical filter 3 receives the series of signal sample 101, the one-line delayed signal 102 and the two-line delayed signal 103. The vertical filter 3 provides a line alternating signal 104 alternating for each line including a chrominance signal. The line alternating signal 104 is applied to a bandpass filter 4. The bandpass filter 4 separates the chrominance signal as a high frequency component from the line alternating signal 104 and provides the chrominance signal 105. The one-line delayed signal 102 and the chrominance signal 105 are supplied to a subtractor 5 . The subtractor 5 separates the chrominance signal 105 from the one-line delayed signal 102 so as to separate and provide a luminance signal 106.
Now, the operation of the luminance signal and chrominance signal separation filter thus structured will be described. First, the operation in case of applying a series of signal sample of a composite color television signal of the NTSC system will be described.
In this case, the series of signal sample 101 of the NTSC composite color television signal sampled in synchronism with the chrominance subcarrier at a sampling frequency f.sub.s =4.multidot.f.sub.sc is display on the screen at positions which are arranged on the screen in a two dimensional lattice as shown in FIG. 6. A delay of one sample and a delay of one line are represented respectively by the characters Z.sup.-1 and Z.sup.-l using a Z-transform. With the conditions of Z.sup.-1 =exp {-jz2.pi.f/4f.sub.sc } and f.sub.sc =455/2f.sub.H, the value l=910 is obtained.
Now, let us assume that a signal sample S (m, n+1), at coordinates (m, n+1) selected out of the respective sample points shown in FIG. 6, is applied to the one-line delay circuit 1. In this case, the one-line delayed signal 102 from the one-line delay circuit 1 is a signal sample (m, n) and the two-line delayed signal 103 from the one-line delay circuit 2 is a signal sample S(m, n-1). These signal samples S(m, n+1), S(m, n) and S(m, n-1) are applied to the vertical filter 3. The vertical filter 3 extracts the line alternating signal including the chrominance signal, alternating for each line. The transfer function H.sub.V (Z) of the vertical filter 3 is H.sub.V (Z)=-1/4(1-Z.sup.-l).sup.2. More specifically stated, the vertical filter 3 extracts the line alternating signal 104 at the coordinates (m, n) on the screen shown in FIG. 6, as H.sub.c (m, n) represented by the below indicated equation (5), regarding the television signals as being similar in the adjacent picture elements. EQU H.sub.c (m, n)=-1/4{S(m, n-1)-2S(m, n)+S(m, n+1)} (5)
Since this line alternating signal H.sub.c (m, n) includes also a luminance signal, a chrominance signal C(m, n) is separated from the line alternating signal H.sub.c (m, n) as a high-frequency component by a horizontal bandpass filter 4. The transfer function H.sub.h (Z) of the horizontal bandpass filter 4 is, for example, H.sub.h (Z)=-1/32(1-Z.sup.-2)(1+Z.sup.-4) (1+Z.sup.-8).
On the other hand, the subtractor 5 receives the one-line delayed signal S(m, n) from the one-line delay circuit 1 and the chrominance signal C(m, n) from the horizontal bandpass filter 4, so that the luminance signal 106 is provided as Y(m, n) represented by the following equation (6): EQU Y(m, n)=S(m, n)-C(m, n) (6)
Now, the operation in case of applying a series of signal samples of a composite color television signal of the PAL system will be described in the following.
In this case, the series of signal sample 101 of a PAL composite color television signal sampled in synchronism with a chrominance subcarrier at a sampling frequency f'.sub.s =4f.sub.sc is arranged on the screen with the phase of the chrominance signal being changed as shown in FIG. 7. More specifically, the phase of the chrominance signal repeats with a period of four lines and as for the even-numbered lines, the chrominance signal component with the phase of the chrominance subcarrier changing by 180.degree. corresponds to the preceding line and as for the odd-numbered lines, the chrominance signal component with the phase changing by 180.degree. corresponds to the succeeding line. Therefore, in order to obtain a PAL four-line alternating signal H'.sub.C corresponding to the NTSC line alternating signal 104 by signal processing in the vertical filter 3, calculating operations are changed alternately for each odd-numbered line and each even-numbered line. The line alternating signals H'.sub.c (m, 2n-1) and H'.sub.c (m, 2n) in this case are represented by the following equations (7) and (8). EQU Odd-numbered line; H'.sub.c (m, 2n-1)=1/2{P(m, 2n-1)-P(m, 2n)}(7) EQU Even-numbered line: H'.sub.c (m, 2n)=1/2{P(m, 2n)-P(m, 2n-1)}(8)
More specifically, using the corresponding upper and lower sample points for every two lines, the line alternating signal 104 is extracted. The line alternating signal 104 thus obtained is supplied to the horizontal bandpass filter 4, so that the chrominance signal 105, that is, C(m, n) is obtained. The chrominance signal 105 is supplied to the subtractor 5 so that the luminance signal Y(m, n) 106 is separated from the one-line delayed signal 102.
Thus, a conventional luminance signal and chrominance signal separation filter is structured by combination of a fixed vertical filter and a fixed horizontal filter and such a conventional separation filter is based on the assumption that picture elements in the series of signal sample of a television signal are similar when the picture elements are adjacent to each other on the screen. Accordingly, such a conventional type of filter has disadvantages that in a region where there are considerable changes in the luminance and the chrominance of the picture, the luminance signal and the chrominance signal are liable to leak to the adjacent channel, causing disturbances in the reproduced picture such as a hue disturbance by a cross color phenomenon or a dot disturbance.
Therefore, an object of the present invention is to provide a dynamic luminance signal and chrominance signal separation filter in which hue disturbances caused by a cross color phenomenon or dot disturbances hardly occur, even in a region where there are considerable changes in the luminance and the chrominance of the picture and thus, disturbance in the reproduced picture can be decreased to a minimum.
Briefly stated, the present invention is a luminance signal and chrominance signal separation filter which operates in the following manner. A series of signal sample of a composite color television signal sampled in synchronism with a chrominance subcarrier at a frequency which is four times the frequency of the chrominance subcarrier is applied and further signal samples are generated at a specified sampled point for separating a luminance signal and a chrominance signal from the series of signal samples and at four or more sample points adjacent thereto on the screen. Based on these signals, a direction is determined in which there is little change in the picture. Based on the determination, two signal samples belonging to a region in which there is little change in the picture are selected and, using these selected signals, the luminance signal and the chrominance signal are separated from the signal samples at the specified sampled point.
According to the present invention, at the time of separating a luminance signal and a chrominance signal at a specified sample point, a direction in which there is little change in the waveform of a composite color television signal is detected using the values of the sample points adjacent to the specified sample point and a luminance signal and a chrominance singal are separated using the sample points positioned in the direction thus detected. Consequently, even in a region where there are considerable changes in the luminance and the chrominance of the picture, a color picture of good quality without cross color or dot disturbance can be reproduced.
These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.